1. Field of the Invention
The invention relates to instrumentation for determining the angular disposition of the sun. In particular, the invention relates to instrumentation for aligning a heliostat so as to provide a stationary light beam output therefrom.
2. Description of the Prior Art
A heliostat is a device known in the prior art. A heliostat, through the use of an automated reflector, tracks the passage of the sun in a manner so as to provide a stationary reflected beam output from the tracking reflector. As with all celestial tracking equipment, the heliostat must be oriented in azimuth and in elevation. The azimuth line of orientation is a true north-sourth meridian. The elevation angle is the polar angle defining the local elevation angle between Earth and the celestial pole. Once established, the azimuthal axis and the polar axis remain as fixed references for the heliostat device. The position of the sun, however, is a variable which must be tracked with reasonable accuracy if a stationary reflected beam output of the heliostat is to result.
The sun appears to move across our skies in an east to west direction. Its apparent movement, however, is not of uniform rate. Therefore, conventional clockwork mechanisms, when used in a solar tracking device, will frequently fall ahead or behind the sun in its apparent traversal of our skies. A second variable is introduced in that the elevation angle of the sun in our sky varies from day to day, season to season. The variation of the sun's elevation angle must also be considered in any solar tracking device.
The solar elevation angle is related to the polar axis by the general term: solar declination. The solar declination is the angular deviation from perpendicularity of the sun's rays with respect to the polar axis. If the sun's rays strike the polar axis at an angle of 90.degree., the solar declination is said to be 0.degree.. This is the situation at the spring equinox. As the sun moves higher in the sky, its rays in their intersection with the polar axis appear to be leaning toward the northern disposition of the polar axis until at the summer solstice the solar declination is said to be about 23.5.degree. north. Having achieved its height at the peak of summer, the sun then begins its descent once more achieving 0.degree. solar declination in the autumn equinox and continuing to decline in the sky with its rays striking the polar axis in a manner which is depicted as though they were leaning toward the southern extreme of the polar axis. As the elevation angle of the sun in the sky continues to decrease, the solar declination increases to approximately 23.5.degree. south at the time of the winter solstice. The cycle is repeated annually with the solar declination moving from south 23.5.degree. to north 23.5.degree. and back to sourth 23.5.degree. each year.
The operation of the heliostat must take into account this variation in the rate of travel of the sun across the sky and the cyclical nature of the solar declination. The heliostat in itself would appear to be a simple device. A mirror is positioned so that the sun's rays striking its surface are reflected at an angle which causes the reflected rays to travel along or parallel to the polar axis. In order to maintain the reflected rays parallel to the polar axis, a clock work mechanism is provided to rotate the mirror as the sun advances across the sky. The mirror is tilted about an axis which is perpendicular to the polar axis and whose projection would lie along an eastwest aximuth line. The mirror tilt angle, however, is not a fixed reference since the mirror must be adjusted to account for the biannual variations in solar declination. So too, an ordinary clock work mechanism will not cause the mirror to faithfully track the sun in its path across the sky since the rate of the sun's travel is not uniform. The basic heliostat, in and of itself, is a relatively inexpensive device. However, if recourse must be had to special, for example computerized, devices for maintaining the alignment of the heliostat with the daily variations in the sun's cycles, its cost rapidly becomes prohibitive. In a general way, the same statement may be made for most devices, such as telescope mounts, which are designed to track the passage of the sun.
It is an object of the invention to provide a relatively inexpensive device which permits the rapid alignment of heliostats and similar equipment permitting the alignment to be made simply and with precision.
A further object of the invention is to provide a solar sighting device which is readily portable and easily positioned for purposes of aligning a heliostat or similar equipment.
It is a more specific objective of the invention to provide a solar sighting device which, while optically sensitive and precise, requires no optical elements.
It is a further specific objective of the invention to provide a solar sight having an elevation scale readout calibrated in terms of solar declination and of heliostat mirror tilt angle.